As regards semiconductor devices, recently, their integration degree thereof is required to increase owing to the demands for a higher operation speed, and thus Cu (copper) is used as an interconnection line metal, because Cu has a low resistivity. Where Cu is used for an interconnection line, the current density is increased, but therefore electro-migration (EM: migration of Cu atoms by an electric current) can easily be caused. The electro-migration may bring about disconnection of the interconnection line and thereby deteriorate the reliability.
In light of this problem, the following method has been proposed to improve the EM resistance of semiconductor devices (for example, see Patent Document 1). Specifically, a plating liquid containing, e.g., CoWB (cobalt tungsten boron) or CoWP (cobalt tungsten phosphorous) is supplied onto the surface of a substrate provided with a Cu interconnection line, so that a metal film of, e.g., CoWB or CoWP, which is called a cap metal, is formed by electroless plating to cover the Cu interconnection line.
In general, a cap metal described above is formed by supplying a plating liquid onto the surface of a substrate provided with a Cu interconnection line. After the cap metal is formed, cleaning of the substrate is performed by use of a cleaning liquid to remove the excess part of the plating liquid deposited on portions of the substrate other than the interconnection line (for example, see Patent Documents 2 and 3).
Typically, in electroless plating performed by use of a plating liquid containing, e.g., CoWB or CoWP, the plating process is promoted by the electromotive force of electrons generated by decomposition of a reducing agent, such as dimethylamineborane (DMAB), which is a derivative of sodium borohydride (SBH) contained in the plating liquid, and hydrogen gas is generated due to decomposition of the reducing agent. If the hydrogen gas brings about bubbles adsorbed on a metal film in the plating process, the plating process cannot proceed at portions corresponding to the bubbles and around the bubbles, so voids are formed in the metal film due to the bubbles and damage the continuity of the metal film. In general, a metal film of, e.g., CoWB or CoWP is poly-crystalline, and thus the film includes crystal grain boundaries and has a low density with large clearances (pinholes) at the crystal grain boundaries.
After the cap metal is formed, slurry-like by-products (residues) generated by the plating reaction are normally present on the surface of the cap metal, and are dried and precipitated in due time. If the precipitated substances are left on the surface of the cap metal, the leakage electric current between interconnection lines may be increased. In order to solve this problem, cleaning of the substrate may be performed to remove the precipitated substances. However, since the precipitated substances strongly stick to the plated surface, the precipitated substances may be difficult to sufficiently remove by the conventional cleaning liquid and cleaning step. Further, the substrate surface may be dried due to deterioration of the wettability relative to a chemical liquid, along with a change in hydrophilic/hydrophobic property. This may cause re-oxidation of the Cu surface.
As described above, the conventional plating method entails difficulties in forming a cap metal of high quality, and thus cannot sufficiently improve the EM resistance of semiconductor devices
[Patent Document 1]
Jpn. Pat. Appln. KOKAI Publication No. 2006-111938
[Patent Document 2]
Jpn. Pat. Appln. KOKAI Publication No. 2000-58487
[Patent Document 3]
Jpn. Pat. Appln. KOKAI Publication No. 2003-179058